The present invention relates to the treatment of a leachant used in leaching impurities from a titaniferous material to upgrade the titania content of the titaniferous material.
The term "titaniferous material" is understood herein to mean a material which contains at least 2 wt % titanium.
In particular the present invention relates to the treatment of a leachant to enhance the effectiveness of the leachant for the removal of impurities in titaniferous materials.
More particularly, although by no means exclusively, the present invention is concerned with minimising the effect on a leaching process of silica and alumina, which are present as impurities in many titaniferous materials.
In a particular embodiment the present invention provides a process whereby the concentrations of silica and alumina in a recycling leachant in a leaching process are maintained below concentrations that affect adversely the leaching process.
In industrial chlorination processes titanium dioxide bearing feedstocks are fed with coke to chlorinators of various designs (fluidised bed, shaft, molten salt), operated to a maxim temperature in the range 700.degree.-1200.degree. C. The most common type of industrial chlorinator is of the fluidised bed design. Gaseous chlorine is passed through the titania and carbon bearing charge, converting titanium dioxide to titanium tetrachloride gas, which is then removed in the exit gas stream and condensed to liquid titanium tetrachloride for further purification and processing.
The chlorination process as conducted in industrial chlorinators is well suited to the conversion of pure titanium dioxide feedstocks to titanium tetrachloride. However, most other inputs (i.e. impurities in feedstocks) cause difficulties which greatly complicate either the chlorination process itself or the subsequent stages of condensation and purification. The attached table provides an indication of the typea of problems encountered. In addition, each unit of inputs which does not enter products contributes substantially to the generation of wastes for treatment and disposal. Some inputs (e.g. heavy metals, radioactives) result in waste classifications which may require specialist disposal in monitored repositories. Preferred inputs to chlorination are therefore high grade materials, with the mineral rutile (at 95-96% TiO.sub.2) the most suitable of present feeds. Shortages of rutile have led to the development of other feedstocks formed by upgrading naturally occurring ilmenite (at 40-60% TiO.sub.2), such as titaniferous slag (approximately 86% TiO.sub.2) and synthetic rutile (variously 92-95% TiO.sub.2). These upgrading processes have had iron removal as a primary focus, but have extended to removal of manganese and alkali earth impurities, as well as some aluminium.
______________________________________ Elemental Input Chlorination Condensation Purification ______________________________________ Fe, Mn Consumes Solid/liquid chlorine, chlorides coke, foul increases ductwork, gas volumes make sludges Alkali Defluidise & alkali earth fluid beds due metals to liquid chlorides, consume chlorine, coke Al Consumes Causes Causes chlorine, corrosion corrosion, coke makes sludges Si Accumulates Can encourage May require in duct distillation chlorinator, blockage. from product reducing Condenses in campaign part with life. titanium Consumes tetrachloride coke, chlorine V Must be removed by chemical treatment and distillation Th, Ra Accumulates in chlorinator brickwork, radioactive; causes disposal difficulties ______________________________________
In the prior art synthetic rutile has been formed from titaniferous minerals, e.g. ilmenite, via various techniques. According to the most commonly applied technique, as variously operated in Western Australia, the titaniferous mineral is reduced with coal or char in a rotary kiln, at temperatures in excess of 1100.degree. C. In this process the iron content of the mineral is substantially metallised. Sulphur additions are also made to convert manganese impurities partially to sulphides. Following reduction the metallised product is cooled, separated from associated char, and then subjected to aqueous aeration for removal of virtually all contained metallic iron as a separable fine iron oxide. The titaniferous product of separation is treated with 2-5% aqueous sulphuric acid for dissolution of manganese and some residual iron. There is no substantial chemical removal of alkali or alkaline earths, aluminium, silicon, vanadium or radionuclides in this process as disclosed or operated. Further, iron and manganese removal is incomplete.
Recent disclosures have provided a process which operates reduction at lower temperatures and provides for hydrochloric acid leaching after the aqueous aeration and iron oxide separation steps. According to disclosures the process is effective in removing iron, manganese, alkali and alkaline earth impurities, a substantial proportion of aluminium inputs and some vanadium as well as thorium. The process may be operated as a retrofit on existing kiln based installations. However, the process is ineffective in full vanadium removal and has little chemical impact on silicon.
In another prior art invention relatively high degrees of removal of magnesium, manganese, iron and aluminium have been achieved. In one such process ilmenite is first thermally reduced to substantially complete reduction of its ferric oxide content (i.e. without substantial metallisation), normally in a rotary kiln. The cooled, reduced product is then leached under 35 psi pressure at 140.degree.-150.degree. C. with excess 20% hydrochloric acid for removal of iron, magnesium, aluminium and manganese. The leach liquors are spray roasted for regeneration of hydrogen chloride, which is recirculated to the leaching step.
In other processes the ilmenite undergoes grain refinement by thermal oxidation followed by thermal reduction (either in a fluidised bed or a rotary kiln). The cooled, reduced product is then subjected to atmospheric leaching with excess 20% hydrochloric acid, for removal of the deleterious impurities. Acid regeneration is also performed by spray roasting in this process. In all of the above mentioned hydrochloric acid leaching based processes impurity removal is similar. Vanadium, aluminium and silicon removal is not fully effective.
In yet another process ilmenite is thermally reduced (without metallisation) with carbon in a rotary kiln, followed by cooling in a nonoxidising atmosphere. The cooled, reduced product is leached under 20-30 psi gauge pressure at 130.degree. C. with 10-60% (typically 18-25%) sulphuric acid, in the presence of a seed material which assists hydrolysis of dissolved titania, and consequently assists leaching of impurities. Hydrochloric acid usage in place of sulphuric acid has been claimed for this process. Under such circumstances similar impurity removal to that achieved with other hydrochloric acid based systems is to be expected. Where sulphuric acid is used radioactivity removal will not be complete.
A commonly adopted method for upgrading of ilmenite to higher grade products is to smelt ilmenite with coke addition in an electric furnace, producing a molten titaniferous slag (for casting and crushing) and a pig iron product. Of the problem impurities only iron is removed in this manner, and then only incompletely as a result of compositional limitations of the process.
A wide range of potential feedstocks is available for upgrading to high titania content materials suited to chlorination. Examples of primary titania sources which cannot be satisfactorily upgraded by prior art processes for the purposes of production of a material suited to chlorination include hard rock (non detrital) ilmenites, siliceous leucoxenes, many primary (unweathered) ilmenites and large anatase resources. Many such secondary sources (e.g. titania bearing slags) also exist.
Clearly there is a considerable incentive to discover methods for upgrading of titaniferous materials which can economically produce high grade products almost irrespectively of the nature of the impurities in the feed.
At present producers of titania pigment by the chloride process require feedstocks to have silica levels as low as possible. In general most feedstocks are less than 2% SiO.sub.2. Where, for various reasons, feedstocks with high levels of silica may be taken in, they are blended against other low silica feedstocks, often with significant cost and productivity penalties. Therefore suppliers of titaniferous feedstocks for chlorination traditionally select ores and concentrates which will result in beneficiated products with low levels of silica. This is generally achieved by mineral dressing techniques based on physical separations. In these processes it is only possible to reject essentially the majority of free quartz particles without sacrificing recovery of the valuable titania minerals. A level of mineralogically entrained silica will normally remain in titaniferous concentrates. In the upgrading processes for ilmenite to synthetic rutile which are presently operated, the removal of iron and other major impurities result in a concentration effect for the silica which exacerbates the requirements for ilmenite concentrates as feedstocks to upgrading plants. Silica is not removed by any commercial upgrading process.
Chemical removal of silica from titaniferous concentrates and upgraded products can be achieved theoretically by aqueous leaching. The leaching of silica from titaniferous materials is frequently accompanied by leaching of other impurities such as alumina. Such impurities are present in most titaniferous materials. When even small concentrations of impurities such as alumina are taken into solution, silica may precipitate as impurity and silica bearing solid compounds within the leach, reducing the effectiveness of the leach. It is therefore necessary to closely control the level of some impurities such as alumina in the leachant produced by any leach treatment process and to employ relatively high ratios of leachant to solid feed.
In the prior art, silica and some other impurities have been removed from titaniferous materials by aqueous leaching with very high excesses of simple caustic solutions. An excess is necessary to prevent impurities present within the titaniferous materials (such as alumina) from interfering with the effectiveness of the leach. In some cases, the spent leachants are discarded. Prior to discard such solutions will generally require neutralisation to satisfy environmental constraints. The cost of the caustic leachant and the neutralisation steps are normally far in excess of the value added to the upgraded titania.
In other processes in which caustic leaching of non titaniferous materials is conducted, spent caustic leachants are regenerated by the addition of lime to precipitate solid calcium silicate compounds which are removed. These processes are conducted in such a manner as to regenerate the active leachant ingredient, viz. caustic, rather than to prevent the accumulation of deleterious impurities observed for titaniferous systems. The precipitates produced frequently have relatively high calcia to silica ratios which will result in much higher consumption of lime than could be economically tolerated for the commensurate silica removal. Further, the precipitates have limited capacity for deleterious impurities. Thus this method of regeneration is of limited usefulness in the treatment of titaniferous materials as deleterious impurities are reported back to the leach step.
In summary the removal of silica and other impurities from titaniferous materials by alkaline leaching is not generally practised due to practical limitations of leachant effectiveness under economic conditions. There is no existing technique in which control of impurities which inhibit the maximum removal of silica is possible while maintaining economic reagent and energy consumption.